Enhancement: Add snapshot summary statistics to rewritten snapshots

The `rewrite` command now supports a `--snapshot-summary` option to add
statistics data to snapshots. Only two fields in the summary will be non-zero:
`TotalFilesProcessed` and `TotalBytesProcessed`.

For snapshots rewritten using the `--exclude` options, the summary
statistics are updated accordingly.

https://github.com/restic/restic/issues/4942
https://github.com/restic/restic/pull/5185